1. Field of the Invention
The present invention relates to a device and a method for producing material having a monocrystalline or multicrystalline structure. The device according to the invention and the method according to the invention are preferably used for the production of monocrystalline silicon.
2. Description of Related Art
A multiplicity of production methods for semiconductor-compatible silicon, particularly for use in photocells, are known from the prior art. For example, the method of melting multicrystalline silicon in a quartz crucible, the Czochralski method and the zonal pulling method are known.
After multicrystalline silicon has been melted in a quartz crucible, slow cooling takes place, which entails disadvantages as regards quality. There are therefore innumerable crystalline regions with different sizes, with the result that the initial product is a multicrystal. The advantage of this melting method is that a large quantity of silicon, for example 800 kg, can be melted in one operation. However, since there is no monocrystal at the end of the crystallization process, preferred semiconductor properties of the resulting multicrystal are not obtained because of the many crystal lattice changes.
In the Czochralski method, which is also designated as a crucible pulling method, silicon is melted in a crucible at a temperature lying slightly above the melting point and a crystal is then pulled out of the melt by turning via a monocrystalline inoculant. This gives rise to a monocrystal having substantially better crystal properties for use in photocells, as compared with the melting of multicrystalline silicon in a quartz crucible.
Both melting in a quartz crucible and the Czochralski method have many disadvantages:
On account of the long dwell time in the quartz crucible, oxygen is released from the quartz crucible in the silicon melt. These oxygen atoms are incorporated into the crystal. When graphite heaters are used for the crucible in order to maintain the temperature, carbon atoms are sublimated into the inert gas atmosphere and are released in the melt. In this case, too, these are incorporated into the crystal. This causes the semiconductor properties to be influenced adversely, with the result that the lifetime of free electrons is seriously reduced, thus causing a reduction in the efficiency of a photocell produced on the basis of this material.
The qualitative disadvantages of the melting method and of the Czochralski method are overcome in the zonal pulling method. The zonal pulling method is also designated as a float zone method.
In the zonal pulling method, heating takes place by induction heating. More specifically, a multicrystalline silicon rod is guided along an induction coil. The silicon rod is remelted from the bottom upward into a monocrystal. The resulting monocrystal has high purity. The advantages of the zonal pulling method are that, in contrast to the Czochralski method and melting method, no adverse material properties with regard to electron lifetime arise. Furthermore, the required energy input is lower because only a limited zone of silicon is melted and substantially lower radiation losses therefore occur.
The disadvantage of the zonal pulling method is that polycrystalline rods of high quality are necessary as initial material. Thus, in the zonal pulling method, the polyrods are subject to high requirements as regards geometrical form and freedom from cracks. A large amount of time has to be spent in order to produce these high-quality polyrods, with the result that high production costs are incurred.
An attempt is made in laid-open publication DE 42 165 19 A1 to link the advantages of the zonal pulling method to the advantages of a process for the beneficial production of multisilicon. More specifically, granular silicon is introduced from above into a reusable silicon tube. This silicon enters the melting zone which closes off the silicon tube and in which heating takes place by induction heating.
The disadvantage of the method according to DE 42 165 19 A1 is that there are stringent requirements for a homogeneous material quality and there has to be a highly controlled material tracking. This material tracking is intended to prevent an uncontrolled escape of the melt and a subsequent possible termination of the entire remelting operation.